The present invention is related in general to the field of semiconductor devices and processes and more specifically to the materials and fabrication of leadframes for integrated circuit devices.
The leadframe for semiconductor devices was invented (U.S. Pat. Nos. 3,716,764 and 4,034,027) to serve several needs of semiconductor devices and their operation simultaneously: First of all, the leadframe provides a stable support pad for firmly positioning the semiconductor chip, usually an integrated circuit (IC) chip. Since the leadframe including the pads is made of electrically conductive material, the pad may be biased, when needed, to any electrical potential required by the network involving the semiconductor device, especially the ground potential.
Secondly, the leadframe offers a plurality of conductive segments to bring various electrical conductors into close proximity of the chip. The remaining gap between the (xe2x80x9cinnerxe2x80x9d) tip of the segments and the conductor pads on the IC surface are typically bridged by thin metallic wires, individually bonded to the IC contact pads and the leadframe segments. Obviously, the technique of wire bonding implies that reliable welds can be formed at the (inner) segment tips.
Thirdly, the ends of the lead segment remote from the IC chip (xe2x80x9couterxe2x80x9d tips) need to be electrically and mechanically connected to xe2x80x9cother partsxe2x80x9d or the xe2x80x9coutside worldxe2x80x9d, for instance to assembly printed circuit boards. In the overwhelming majority of electronic applications, this attachment is performed by soldering. Obviously, the technique of soldering implies that reliable wetting and solder contact can be performed at the (outer) segment tips.
It has been common practice to manufacture single piece leadframes from thin (about 120 to 250 xcexcm) sheets of metal. For reasons of easy manufacturing, the commonly selected starting metals are copper, copper alloys, iron-nickel alloys (for instance the so-called xe2x80x9cAlloy 42xe2x80x9d), and invar. The desired shape of the leadframe is etched or stamped from the original sheet. In this manner, an individual segment of the leadframe takes the form of a thin metallic strip with its particular geometric shape determined by the design. For most purposes, the length of a typical segment is considerably longer than its width.
In the European patent No. 335 608 B1, issued Jun. 14, 1995 (Abbott, xe2x80x9cLeadframe with Reduced Corrosionxe2x80x9d), U.S. Pat. No. 6,194,777, issued Feb. 27, 2001 (Abbott, xe2x80x9cLeadframes with Selective Palladium Platingxe2x80x9d), and No. 6,246,446, issued Jun. 12, 2001 (Abbott, xe2x80x9cLeadframe with Reduced Corrosionxe2x80x9d), a palladium-plated leadframe is introduced which is not subject to corrosion due to galvanic potential forces aiding the migration of the base metal ions to the top surface where they will form corrosion products. The patent describes a sequence of layers consisting of nickel (over the base metal), palladium/nickel alloy, nickel, and palladium (outermost). This technology has been widely accepted by the semiconductor industry.
After assembly on the leadframe, most ICs are encapsulated, commonly by plastic material in a molding process. It is essential that the molding compound, usually an epoxy-based thermoset compound, has good adhesion to the leadframe and the device parts it encapsulates. Palladium, described above as the outermost layer of the leadframe, offers excellent adhesion to molding compounds.
Nickel/palladium plated leadframes are used because of their low total cost of ownership, primarily a result of eliminating post-mold solder plating. However, some customers, for instance automotive manufacturers and telephone central switching offices, require solder plated external leads, typically because of burn-in, accelerated testing or environmental conditions.
If solder dipping is used after molding, the palladium will dissolve into the solder and the nickel is then solderable. However, solder dipping is not practical for devices with fine-pitch leadframes because of solder bridging.
The price of palladium climbed in the last decade to approximately twice the gold price. Cost reduction pressures in semiconductor manufacturing have initiated efforts to reduce the thickness of the palladium layers employed to about one third of its previous thickness. At this thinness, palladium does not prevent oxidation of the underlying nickel which will inhibit its solderability.
In U.S. patent applications Ser. No. 60/138,070, filed on Jun. 8, 1999 (Abbott, xe2x80x9cPalladium-Spot Leadframes for Solder Plated Semiconductor Devices and Method of Fabricationxe2x80x9d), and No. 60/214,314, filed on Jun. 27, 2000 (Abbott, xe2x80x9csemiconductor Leadframes Plated with Lead-Free Solder and Minimum Palladiumxe2x80x9d), to which the present invention is related, a fabrication process for palladium layers of reduced thickness is described. There is, however, a problem for pre-plated leadframes in the selection of a tin-based solder without the risk of tin dendrite growth. Further, there is a severe discrepancy between the high speed and throughput of nickel and palladium plating for the desired thicknesses, and the roughly 20xc3x97 slower tin plating for the desired thickness.
An urgent need has therefore arisen for a low-cost, reliable mass production method for a leadframe combining the advantages of palladium with its bondability and adhesion capability to molding compounds, and the application of pre-plated tin solder. The palladium layer should have reduced thickness. The leadframe and its method of fabrication should be flexible enough to be applied for different semiconductor product families and a wide spectrum of design and assembly variations, and should achieve improvements toward the goals of improved process yields and device reliability. Preferably, these innovations should be accomplished using the installed equipment base so that no investment in new manufacturing machines is needed.
The present invention describes a method for fabricating a leadframe structure comprising a chip mount pad and a plurality of lead segments, each having a first end near the mount pad and a second end remote from said mount pad. The structure is formed from a sheet-like starting material. In a first plating system, the leadframe is plated with a layer of nickel. Next, the second segment ends are selectively masked and a layer of palladium is selectively plated on the nickel layer on the exposed chip pad and first segments ends in a thickness suitable for wire bonding attachment. In a second plating system, the chip pad and first segment ends are selectively masked and a pure tin layer is selectively plated on the nickel layer on the exposed second segment ends in a thickness suitable for parts attachment.
The present invention is related to high density ICs, especially those having high numbers of inputs/outputs, or contact pads, and also to devices in packages requiring surface mount in printed circuit board assembly. These ICs can be found in many semiconductor device families such as standard linear and logic products, digital signal processors, microprocessors, digital and analog devices, and both large and small area chip categories. The invention represents a significant cost reduction and enhances environmental protection and assembly flexibility of semiconductor packages, especially the plastic molded packages, compared to the conventional copper-based post-mold-plated leadframes.
It is an aspect of the present invention to provide a technology for enabling solder package leads with pre-plated pure tin, while maintaining a palladium layer in the localized areas intended for wire bonding with its significant cost advantage over the traditional silver spot plated inner leads.
Another aspect of the invention is to provide the tin as a matte, coarse grain, low carbon content deposit of relatively high thickness (about 4 to 6 xcexcm). The tin annealing is a by-product of the molding compound curing after encapsulation. Tin whiskers on the outside of the package are suppressed by the nickel.
Another aspect of the invention is to reach these goals with a low-cost manufacturing method without the cost of equipment changes and new capital investment, by using the installed fabrication equipment base.
The invention utilizes a first, wheel-based plating system to deposit the nickel and palladium layers, and a second, flood cell system to deposit the tin layer.
Another aspect of the invention is to produce leadframes so that established wire bonding processes can continue unchanged, and that established board attachment process can continue unchanged.
Another aspect of the invention is to eliminate silver and the cyanide solution used for its deposition from the leadframe manufacturing process flow, resulting in less costly waste treatment.
Another aspect of the invention is to introduce a palladium spot plating technology with provides loose tolerance for the spot boundaries, thus simplifying leadframe manufacturing and lowering fabrication cost.
These aspects have been achieved by the teachings of the invention concerning deposition and masking methods suitable for mass production. Various modifications of leadframe preparations have been successfully employed.
In the preferred embodiment of the invention, a plated layer of nickel is fully covering the leadframe base material. A plated layer of pure tin is plated onto the nickel layer so that it covers selectively leadframe areas intended for parts attachment, especially board assembly. A layer of palladium is then plated onto the nickel layer so that it covers selectively the leadframe areas intended for bonding wire attachment.
Leadframes prepared according to the invention can be successfully used in surface mount technologies based on bending the package lead segments.
The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.